feat: add calendar widget components

Add date picker and event calendar components inspired by shadcn/radix
Calendar and org-mode-agenda-cli.

Components:
- ui.calendar: Month grid date picker with navigation, today/selected
  highlighting, outside-month dimming. Pure date math utilities
  (days-in-month, day-of-week, calendar-days, etc.)
- ui.calendar-events: Event-aware grid with colored pills, horizontal
  day ticker strip with dot indicators, and agenda list view with
  grouped events by day

CSS: Token-based styling with dark mode support for event color
variants (accent/danger/success/warning). Responsive breakpoints.

All three targets supported (squint/cljs/clj). Dev pages show calendar
on its own page with interactive demos (date selection, month nav,
event grid, ticker, agenda list).

Tests: 27 new assertions covering date math, class generation,
component structure, event filtering/sorting.

bb.edn

@@ -48,6 +48,8 @@
               [ui.sidebar-test]
               [ui.chip-test]
               [ui.separator-test]
+              [ui.calendar-test]
+              [ui.calendar-events-test]
               [ui.theme-test])
    :task (let [{:keys [fail error]} (t/run-tests
                                       'ui.button-test
@@ -69,6 +71,8 @@
                                       'ui.sidebar-test
                                       'ui.chip-test
                                       'ui.separator-test
+                                      'ui.calendar-test
+                                      'ui.calendar-events-test
                                       'ui.theme-test)]
            (when (pos? (+ fail error))
              (System/exit 1)))}